Although coil springs have recently come into favor for passenger car suspension systems, leaf springs remain in favor for use particularly in truck suspension systems and are likely to for many years to come because of the load capacity, packaging and axle location advantages associated therewith. Although described herein with particularity to leaf springs, the present invention includes any vehicle suspension member subject to torsional or bending loads and having a longitudinal axis extending between opposite ends such as torque rods (also called trailing arms) for axle control; traction bars (keep axle from twisting during braking); and track rods (keep axle from shifting laterally).
Leaf springs generally comprise a singular leaf or plate or a plurality of leafs or plates of substantially constant or tapered thickness most commonly made from steel and secured together and operative to support the frame of the vehicle on the wheel axle. As might be expected, the number of leafs or plates employed in the leaf spring is based on allowable design stress for a given load capacity and deflection range.
Each plate of a leaf spring is subjected to cantilever bending producing a (positive) tensile stress on the upper, typically concave, surface of the leaf to which the vehicle load or force is applied. As a result, a balancing (negative) compressive stress is developed on the lower, typically convex, surface of the leaf. The tensile stress is a maximum at the upper surface of the leaf and decreases to zero at or near the center of the plate thickness also known as the neutral axis hereinafter referred to as the longitudinal axis extending between opposite ends of the plate or leaf. Similarly, the balancing compressive stress is a maximum on the lower surface declining to zero at the longitudinal axis. Virtually all cracks are initiated at or near the upper surface of the leaf spring plate at the point of maximum tensile stress.
Due to high strength requirements, leaf springs must have high hardness which causes them to be prone to rapid crack propagation through the leaf cross-section once a relatively shallow crack is developed at the tensile surface. The incidence of crack initiation and progression is generally a function of increased service life or time and also service conditions including load history and corrosion. No matter how a crack may develop, i.e., fatigue and/or corrosion, its progression beyond a critical depth of generally less than one-half of section thickness is rapid to complete fracture or separation. The present invention provides a means of diverting crack propagation which initiates at or near the upper tensile surface and progresses through the thickness of the leaf or plate in a direction generally perpendicular to the longitudinal axis of the leaf by redirecting crack progression to a direction generally parallel thereto. Effectively, the parallel crack or delamination of the flat (or tapered) plate section lowers section stiffness causing the spring to sag well before final fracture providing the user some early indication of impending spring failure prior to complete separation and possible vehicle debilitation.